How do atomic clocks work?

15 May 2025

Since 1967, atomic clocks have been defining the length of a second and, thus, the measurement of time throughout the world. But how do atomic clocks operate, and what exactly makes them so special? 

If you’re on a German railway platform waiting for a train and your gaze happens to wander to the station clock, you will notice something odd: Every time the red second hand has made a full lap around the dial, it stops for a moment – only to suddenly jump forward two seconds and start a new round. The station timepiece takes this little break because it is waiting for the signal from a mother clock, which in turn refers to an atomic clock to ensure that it is displaying the correct time – all the time.

Germany’s time is made in Braunschweig: in the clock hall of the Physikalisch-Technische Bundesanstalt, to be precise. This is a gym-sized room whose walls are lined with copper so that no disruptive signal from the outside can affect the accuracy of the atomic clocks that work tirelessly to keep the nation’s time.

In principle, these atomic clocks work like an ordinary clock. They have an impulse generator and a counter to count the beats. But unlike a grandfather clock, it is not the oscillation of a pendulum that sets the pace, but the oscillations of an electron in an atom. A caesium atom, to be precise. This alkali metal is characterised by the fact that it emits very precise oscillations when switching between two energy states.

First, the caesium is evaporated in a furnace, then its atoms are sorted by a magnetic field: Excited caesium atoms in a more highly energetic state are “screened out”; only atoms in the low energetic state are allowed to pass through and enter a microwave resonator. Here, the atoms are irradiated by a microwave field, prompting them to change their state. Then the next selection takes place: At this point, the “unanimated” atoms are weeded out, and only the atoms that have changed their state are captured. Since the number of atoms captured is greatest when the microwave field is at a certain frequency, it is this frequency that is maintained for the count to take place: Once 9,192,631,770 oscillations have been counted, one second has elapsed. These nine billion oscillations have been the international definition of a second since 1967.

Whereas a quartz clock deviates from real time by a few seconds per month, the CS2 caesium atomic clock in Braunschweig only loses or gains one second in three million years. The caesium fountain clock CSF2 does even better, deviating by just one second – if that – in 30 million years. This is one of the primary clocks that produces the time for the railway stations, radio-controlled clocks and time announcements in Germany.

What makes fountain clocks so special? While caesium atoms travel at around 300 metres per second at room temperature, they are cooled very abruptly in the approximately two-metre-high metal cylinder of the fountain clock, throttling them back to a speed of one centimetre per second. They are then shot upwards like the water drops in a fountain. During their flight, the atoms are irradiated with microwaves to bring them into the higher energetic state. Because these atoms are now travelling so extraordinarily slowly, the interaction time with the microwaves is extended. Measuring them thus provides particularly precise results.

There are over 400 large atomic clocks worldwide that are networked with each other and set the same rhythm everywhere. While such incredible accuracy was not really required for thousands of years, the situation is different in our high-tech present. Without atomic clocks, cars, ships and planes would be unable to find their way using GPS satellites, and it would be impossible for the first automated tractors to stay on track.

GPS satellites constantly transmit their current position and time. The GPS receiver in our navigation system or smartphone picks up these signals and can use the difference between arrival and transmission time to calculate how long the signal has been travelling and how far it has come. The receiver then uses the signals from several satellites to determine its position on Earth.

How accurate the satellite geolocation turns out to be depends on how precisely the transit time of the GPS signals can be calculated. And for this to work, the signals’ transmission and arrival times must be measured as accurately as possible, because even a deviation of one millisecond will throw the calculated position off by 300 kilometres. GPS satellites therefore have their own small atomic clock on board. Because it would be virtually impossible to accommodate such a clock in the GPS receivers of our smartphones, they must instead listen out for the signal from a fourth satellite for time correction in addition to three satellites for geolocation.

But this is far from everything that atomic clocks are called upon to do. The military uses them to for submarine and drone navigation, oil companies to find new deposits, researchers to make high-precision measurements, and financial whizzkids to prompt high-performance computers to independently buy or sell securities within seconds in what is known as high-frequency trading.

And atomic clocks also help to satisfy our ever-increasing hunger for data. To transmit large amounts of data in a brief period during streaming or video conferences, the sender and receiver must be in sync to prevent everything from getting mixed up. It’s for this reason that Deutsche Telekom, for example, operates its own atomic clock to keep its systems in rhythm. However, atomic clocks also help to keep the frequency of our power grids stable; the ever-growing use of renewables means that these grids are subject to greater fluctuations than used to be the case.

Research is already underway worldwide into the next level of accuracy, looking at clocks in which the atoms are irradiated with laser light instead of microwaves, thus dividing the second into much smaller units. The optical atomic clock which was developed at the Physikalisch-Technische Bundesanstalt in Braunschweig ticks trillions of times per second. If it had been beating time for the approximately 14 billion years that have passed since the Big Bang, it would now be only one second out. An optical atomic clock presented by US researchers in 2024 is reported to be so accurate that it would only deviate from real time by one second every 30 billion years.

These optical atomic clocks are still in the testing phase. Once they are ready for continuous operation, their incredible accuracy should help to clarify certain fundamental questions in physics, measure the Earth more precisely and further improve GPS systems. Looking forward, it should then be possible to determine the location of people and machines down to the nearest millimetre. This will be a real quantum leap, especially for aviation and self-driving cars.